1. Field of the Invention
The present invention is in the field of semiconductor wafer processing, and more specifically relates to a disposable polishing pad for use in a chemical mechanical polishing operation performed on the semiconductor wafers wherein the polishing pad contains an optical sensor for monitoring the condition of the surface being polished while the polishing operation is taking place to permit determination of the endpoint of the process.
2. The Prior Art
In U.S. Pat. No. 5,893,796 issued Apr. 13, 1999 and in continuation Pat. No. 6,045,439 issued Apr. 4, 2000, Birang et al. show a number of designs for a window installed in a polishing pad. The wafer to be polished is on top of the polishing pad, and the polishing pad rests upon a rigid platen so that the polishing occurs on the lower surface of the wafer. That surface is monitored during the polishing process by an interferometer that is located below the rigid platen. The interferometer directs a laser beam upward, and in order for it to reach the lower surface of the wafer, it must pass through an aperture in the platen and then continue upward through the polishing pad. To prevent the accumulation of slurry above the aperture in the platen, a window is provided in the polishing pad. Regardless of how the window is formed, it is clear that the interferometer sensor is always located below the platen and is never located in the polishing pad.
In U.S. Pat. No. 5,949,927 issued Sep. 7, 1999 to Tang, there are described a number of techniques for monitoring polished surfaces during the polishing process. In one embodiment Tang refers to a fiber-optic cable embedded in a polishing pad. This cable is merely a conductor of light. The light source and the detector that do the sensing are located outside of the pad. Nowhere does Tang suggest including a light source and a detector inside the polishing pad. In some of Tang""s embodiments, fiber-optic decouplers are used to transfer the light in the optical fibers from a rotating component to a stationary component. In other embodiments, the optical signal is detected onboard a rotating component, and the resulting electrical signal is transferred to a stationary component through electrical slip rings. There is no suggestion in the Tang patent of transmitting the electrical signal to a stationary component by means of radio waves, acoustical waves, a modulated light beam, or by magnetic induction.
In another optical end-point sensing system, described in U.S. Pat. No. 5,081,796 issued Jan. 21, 1992 to Schultz there is described a method in which, after partial polishing, the wafer is moved to a position at which part of the wafer overhangs the edge of the platen. The wear on this overhanging part is measured by interferometry to determine whether the polishing process should be continued.
In conclusion, although several techniques are known in the art for monitoring the polished surface during the polishing process, none of these techniques is entirely satisfactory. The fiber optic bundles described by Tang are expensive and potentially fragile; and the use of an interferometer located below the platen, as used by Birang et al., requires making an aperture through the platen that supports the polishing pad. Accordingly, the present inventor set out to devise a monitoring system that would be economical and robust, taking advantage of recent advances in the miniaturization of certain components.
It is an objective of the present invention to provide a polishing pad in which an optical sensor is contained, for monitoring an optical characteristic, such as the reflectivity, of a wafer surface that is being polished, during the polishing operation. The real-time data derived from the optical sensor enables, among other things, the end point of the process to be determined.
It is a further objective of the present invention to provide apparatus for supplying electrical power to the optical sensor in the polishing pad.
It is a further objective of the present invention to provide apparatus for supplying electrical power for use in transmitting an electrical signal representing the optical characteristic from the rotating polishing pad to an adjacent non-rotating receiver.
It is a further objective of the present invention to provide a disposable polishing pad containing an optical sensor, wherein the polishing pad is removably connectable to a non-disposable hub that contains power and signal processing circuitry.
In accordance with the present invention, an optical sensor that includes a light source and a detector is disposed within a blind hole in the polishing pad so as to face the surface that is being polished. Light from the light source is reflected from the surface being polished and the reflected light is detected by the detector which produces an electrical signal related to the intensity of the light reflected back onto the detector.
The electrical signal produced by the detector is conducted radially inward from the location of the detector to the central aperture of the polishing pad by a thin conductor concealed between the layers of the polishing pad.
The disposable polishing pad is removably connected, both mechanically and electrically, to a hub that rotates with the polishing pad. The hub contains electronic circuitry that is concerned with supplying power to the optical sensor and with transmitting the electrical signal produced by the detector to non-rotating parts of the system. Because of the expense of these electronic circuits, the hub is not considered to be disposable. After the polishing pad has been worn out from use, it is disposed of, along with the optical sensor and the thin conductor.
In accordance with the present invention, electrical power for operating the electronic circuits within the hub and for powering the light source of the optical sensor may be provided by several techniques. In a preferred embodiment, the secondary winding of a transformer is included within the rotating hub and a primary winding is located on an adjacent non-rotating part of the polishing machine. In a first alternative embodiment, a solar cell or photovoltaic array is mounted on the rotating hub and is illuminated by a light source mounted on a non-rotating portion of the machine. In another alternative embodiment, electrical power is derived from a battery located within the hub. In yet another embodiment, electrical conductors in the rotating polishing pad or in the rotating hub pass through the magnetic fields of permanent magnets mounted on adjacent non-rotating portions of the polishing machine, to constitute a magneto.
In accordance with the present invention, the electrical signal representing an optical characteristic of the surface being polished is transmitted from the rotating hub to an adjacent stationary portion of the polishing machine by any of several techniques. In a preferred embodiment, the electrical signal to be transmitted is used to frequency modulate a light beam that is received by a detector located on adjacent non-rotating structure. In alternative embodiments, the signal is transmitted by a radio link or an acoustical link. In yet another alternative embodiment, the signal may be applied to the primary winding of a transformer on the rotating hub and received by a secondary winding of the transformer located on an adjacent non-rotating portion of the polishing machine. This transformer may be the same transformer that is used for coupling electrical power into the hub, or it can be a different transformer.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.